1. Field of the Invention
The present invention relates to a positron imaging apparatus for determining an internal substance distribution in a body to be measured.
2. Related Background Art
Attention is being drawn to positron imaging apparatus for determining the internal substance distribution in a body to be measured. With the positron imaging apparatus, chemical compounds labeled with radioisotopes to emit positrons are put into the body to be measured such as a human body or an animal body, and measurement is made for a pair of .gamma.-rays which are produced when (pair) annihilation between a positron emitted from the radioisotope annihilates combining with an electron in an ordinary substance, whereby the internal substance distribution is determined in the measured body. Energy of each .gamma.-ray in the annihilation is approximately the same as the mass energy (0.511 MeV) of a positron or an electron. The two .gamma.-rays are emitted in opposite directions to each other. A distribution of the labeled substances inside the measured body is determined by measuring such the annihilation .gamma.-ray pairs within the measured body. In view of the energy of each .gamma.-ray and the present status of measuring means, a general measuring system is one for letting the photon emit scintillation in a .gamma.-ray detecting element (so called scintillator) and thereafter detecting the scintillation with a photodetector.
Among apparatus employing the above method is a positron CT apparatus (or PET (Positron Emission Tomography) apparatus). Such a PET apparatus is so constructed that a number of .gamma.-ray detectors each composed of a .gamma.-ray detecting element (for example, a BGO scintillator) and a photodetector are arranged in multi-layer ring structure around a certain axis. When two .gamma.-ray detectors detect .gamma.-rays at the above specific energy (0.511 MeV) in coincidence, an event of positron annihilation is recognized. Spatially connecting between the coincidence detectors can give a presumable line on which the position of annihilation is located. The substance distribution in the measured body is determined by obtaining a number of presumable lines for annihilation and performing arithmetic processing.
There are also reports on positron probe apparatus (hereinafter referred to as TOF positron probe) utilizing a time of flight of .gamma.-ray as an apparatus employing the method for detecting two .gamma.-rays accompanying an event of positron-electron annihilation, which measures a difference between .gamma.-ray arrival times at two detectors for detecting two annihilation .gamma.-rays (M. Yamamoto et al.: IEEE TRANSACTION ON NUCLEAR SCIENCE, Vol. 36, No. 1, 1989, pp 998-1002, for example). The TOF positron probe determines the substance distribution in a measured body by obtaining a lot of presumable annihilation events and performing arithmetic processing.